1. Field of the Invention
The present invention relates generally to a demodulator circuit in a global positioning system receiver used in a satellite navigation field, and more particularly to a demodulator circuit in which a position of an artificial satellite is measured by using a radio wave transferred from the satellite.
2. Description of the Prior Art
Recently, a global positioning system receiver in which a radio wave transferred from a position-measuring satellite is received to measure an absolute position of the satellite has been broadly utilized as a navigation apparatus in a mobile, ship or aircraft. For example, a conventional global positioning system receiver is disclosed in Published Unexamined Japanese Patent Application No. 103487 of 1990 (H2-103487).
2.1. PREVIOUSLY PROPOSED ART
FIG. 1 is a block diagram of a conventional demodulator circuit of a global system receiver in which 32 types of signals in 8 channels (or eight satellites) are demodulated.
As shown in FIG. 1, a conventional demodulator circuit 11 is composed of an antenna 12 for receiving eight types of radio waves transferred from eight position-measuring satellites (NAVSTAR) 13, an amplifier 14 for amplifying the radio waves in a group to form an amplified signal, a first local oscillator 15 for generating a first oscillating signal, a mixing unit 16 for mixing the amplified signal with the first oscillating signal to convert in frequency the amplified signal into an intermediate-frequency signal, an intermediate frequency amplifier 17 for amplifying the intermediate-frequency signal, a second local oscillator 18 for generating an in-phase oscillating signal and an orthogonal oscillating signal orthogonal to each other, a first orthogonal frequency converter 19a for orthogonal-converting the amplified intermediate-frequency signal with the in-phase oscillating signal to form an in-phase signal Si, a second orthogonal frequency converter 19b for orthogonal-converting the amplified intermediate-frequency signal with the orthogonal oscillating signal to form an orthogonal signal Sq, thirty-two pseudo-noise code generators 20 for respectively generating one of four phase-types of pseudo-noise code signals corresponding to a noise code peculiar to one of the eight position-measuring satellites 13, sixty-four correlation units 21 for generating thirty-two in-phase correlation signals indicating degrees of correlation between the in-phase signal Si and the thirty-two pseudo-noise code signals and thirty-two orthogonal correlation signals indicating degrees of correlation between the orthogonal signal Sq and the thirty-two pseudo-noise code signals, sixty-four filters 22 for filtering the thirty-two in-phase correlation signals and the thirty-two orthogonal correlation signals, eight carrier local oscillators 23 for respectively generating an in-phase carrier oscillating signal and an orthogonal carrier oscillating signal orthogonal to each other corresponding to one of eight carrier waves of the radio waves transferred from the position-measuring satellites 13, thirty-two third orthogonal frequency converters 24 for receiving thirty-two sets of correlation signals respectively composed of an in-phase correlation signal and an orthogonal correlation signal filtered in the filters 22 and respectively converting in frequency one of the thirty-two sets of correlation signals into an in-phase base signal and an orthogonal base signal orthogonal to each other according to one set of in-phase carrier oscillating signal and orthogonal oscillating signal generated in one of the carrier local oscillators 23, sixty-four integrating units 25 for time-integrating the thirty-two in-phase base signals and the thirty-two orthogonal base signals each time the pseudo-noise code signals are generated in the pseudo-noise code generators 20 to form sixty-four integrating signals indicating integrated values, a control unit 26 for controlling code phases of the pseudo-noise code signals generated in the pseudo-noise code generators 20 and frequencies of the carrier oscillating signals generated in the carrier local oscillators 23 according to the integrating signals to track the noise codes and the carrier waves peculiar to the eight position-measuring satellites 13, a measured position calculating unit 27 for calculating a position of the antenna 12 from eight pieces of orbital information and eight pieces of time information of the position-measuring satellites 13 included in the integrating signals and an outputting unit 28 for outputting the position of the antenna 12.
In the above configuration, an operation performed in the conventional demodulator circuit 11 is described.
A noise code and satellite signals indicating a piece of orbital information and a piece of time information are output with a carrier wave from each position-measuring satellite 13 as a radio wave. That is the carrier wave is modulated by the noise code peculiar to each of the satellites 13 and the satellite signal, and an energy of the radio wave is diffused.
When the radio waves of a plurality of position-measuring satellite 13 are received in the antenna 12 of a movable body such as a mobile, ship or aircraft, the radio waves are Doppler-shifted. The radio waves received in the antenna 12 are amplified in the amplifier 14 in a group to form an amplified signal. Thereafter, the amplified signal is mixed in the mixing unit 16 with the first oscillating signal generated in the first local oscillator 15 to convert in frequency the amplified signal into an intermediate-frequency signal. Thereafter, the intermediate-frequency signal is amplified in the intermediate frequency amplifier 17. Thereafter, the amplified intermediate-frequency signal is orthogonal-converted with the in-phase oscillating signal in the first orthogonal frequency converter 19a to form an in-phase signal Si. Also, the amplified intermediate-frequency signal is orthogonal-converted with the orthogonal oscillating signal in the second orthogonal frequency converter 19b to form an orthogonal signal Sq.
In the pseudo-noise code generators 20, four phase-types of pseudo-noise code signals corresponding to a noise code peculiar to one position-measuring satellite 18 are generated for each of the position-measuring satellites 13. Therefore, thirty-two pseudo-noise code signals are generated in the thirty-two pseudo-noise code generators 20 in one-to-one correspondence. Code phases of the pseudo-noise code signals corresponding to a noise code are close to those of the corresponding noise code under the control of the control unit 26. Thereafter, thirty-two in-phase correlation signals indicating degrees of correlation between the in-phase signal Si and the thirty-two pseudo-noise code signals are generated in thirty-two correlation units 21. Also, thirty-two orthogonal correlation signals indicating degrees of correlation between the orthogonal signal Sq and the thirty-two pseudo-noise code signals are generated in the other thirty-two correlation units 21. In this case, the stronger a degree of correlation, the lower a frequency of an in-phase or orthogonal correlation signal relating to the correlation. Also, because the pseudo-noise code signals generated in the thirty-two pseudo-noise code generators 20 correspond to the noise codes of the position-measuring satellites 18, the noise codes included in the in-phase signal Si and the orthogonal signal Sq are removed in the correlation units 21. Thereafter, the orthogonal correlation signals and the in-phase correlation signals are filtered in the filters 22 in one-to-one correspondence to pass orthogonal correlation signals and in-phase correlation signals respectively having a low frequency. That is, an intensity of an in-phase or orthogonal correlation signal indicating a weak correlation is reduced in the filter 22, and an intensity of an in-phase or orthogonal correlation signal indicating a strong correlation are maintained at a high value in the filter 22.
In each of the carrier local oscillators 23, an in-phase carrier oscillating signal and an orthogonal carrier oscillating signal which are orthogonal to each other and correspond to one of eight carrier waves of the radio waves transferred from the position-measuring satellites 13 are generated. That is, a frequency of the in-phase and orthogonal carrier oscillating signals is close to that of a corresponding carrier wave under the control of the control unit 26. Thereafter, the thirty-two sets of correlation signals respectively composed of an in-phase correlation signal and an orthogonal correlation signal filtered in the filters 22 are input to the third orthogonal frequency converters 24 in one-to-one correspondence. Also, a set of the in-phase carrier oscillating signal and the orthogonal carrier oscillating signal corresponding to a position-measuring satellite 13 is transferred from each carrier local oscillator 23 to four third orthogonal frequency converters 24 in which four correlation signals relating to the corresponding position-measuring satellite 13 are input. Thereafter, in each of the third orthogonal frequency converters 24, a set of correlation signals is converted in frequency into an in-phase base signal and an orthogonal base signal orthogonal to each other according to one set of in-phase carrier oscillating signal and orthogonal oscillating signal. Because the frequency of the in-phase and orthogonal carrier oscillating signal is close to that of a corresponding carrier wave, the carrier wave included in each of the correlation signals is removed in the converters 24.
Thereafter, the thirty-two in-phase base signals and the thirty-two orthogonal base signals are time-integrated in the integrating units 25 in one-to-one correspondence each time the pseudo-noise code signals are generated in the pseudo-noise code generators 20, and sixty-four integrating signals indicating integrated values of the in-phase and orthogonal base signals are output to the control unit 26. In the control unit 28, code phases of the pseudo-noise code signals generated in the-pseudo-noise code generators 20 and frequencies of the carrier oscillating signals generated in the carrier local oscillators 23 are controlled according to the integrating signals to track the noise codes and the carrier waves peculiar to the eight position-measuring satellites 13. In other words, in cases where a code phase of a noise code in each position-measuring satellite 13 almost agrees with one of four code phases of the four pseudo-noise code signals corresponding to the position-measuring satellite 13 on condition that a frequency of a carrier wave in each position-measuring satellite 13 almost agrees with that of in-phase and orthogonal carrier oscillating signals generated in the carrier local oscillator 23, the satellite signals indicating the orbital information and the time information in each position-measuring satellite 13 are detected in the control unit 26. Therefore, when the code phases of the pseudo-noise code signals generated in the pseudo-noise code generators 20 and the frequencies of the in-phase and orthogonal carrier oscillating signals generated in the carrier local oscillator 23 almost agree with the code phases of the noise codes and the frequencies of the carrier waves in the position-measuring satellites 13 under the control of the control unit 26, the satellite signals detected are demodulated in the control unit 26 and are transferred to the measured position calculating unit 27.
In the unit 27, a position of the antenna 12 is calculated from eight pieces of orbital information and eight pieces of time information of the position-measuring satellites 13. Thereafter, the position of the antenna 12 is output from the outputting unit 28.
Accordingly, because the code phases of the pseudo-noise code signals generated in the pseudo-noise code generators 20 and the frequency of the in-phase and orthogonal carrier oscillating signals generated in the carrier local oscillator 23 are adjusted under the control of the control unit 26 to track the noise codes and the carrier waves peculiar to the eight position-measuring satellites 13, the satellite signals indicating the orbital information and the time information can be detected, and the position of the antenna 12 can be determined.
Also, because the four phase-types of the in-phase and orthogonal pseudo-noise code signals generated in the four pseudo-noise code generators 20 correspond to one code phase of one position-measuring satellite 13, the satellite signals of one position-measuring satellite 13 are scanned in a parallel processing to make one of the four phase-types agree with the code phase of one position-measuring satellite 13. Therefore, the satellite signals of each position-measuring satellite 13 can be quickly detected. That is, the position of the antenna 12 can be quickly determined.
2.2. PROBLEMS TO BE SOLVED BY THE INVENTION
However, in cases where a plurality of pseudo-noise code generators 20 are arranged for each of the position-measuring satellites 13, the number of correlation units 21 and the number of filters 22 are extremely increased. In this case, because many multipliers are required for each correlation unit 21, a circuit size of the conventional demodulator circuit 11 is enlarged, an electric power consumed in the conventional demodulator circuit 11 is increased, and a manufacturing cost of the conventional demodulator circuit 11 is increased.
Also, because only one carrier local oscillator 23 is arranged for each of the position-measuring satellites 13, the removal of the carrier wave cannot be quickly performed.
In addition, in a second conventional art, the in-phase and orthogonal correlation signals generated in the correlation units 21 are transformed according to a fast Fourier transformation, and the satellite signals of the position-measuring satellites 13 are detected. However, though the satellite signals of a plurality of position-measuring satellites 13 can be quickly detected, the processing of the fast Fourier transformation is complicated, and a manufacturing cost of a conventional demodulator circuit according to the second prior art is increased.